1. Field of the Invention
This invention relates to apparatus for reading disk-image data from a streaming tape drive system and, more particularly, to a system for removing unwanted frequency modulation from an output data stream.
2. Discussion of the Related Art
Streaming cartridge tape drives provide mass storage of magnetic information by writing long streams of serial data pulses in a plurality of parallel streams on magnetic tape. Each stream is written at a different vertical position on the tape. After completing one serial data stream along the entire tape length at one vertical position on the tape, the streaming tape drive reverses tape direction and writes another serial data stream at a second vertical position on the tape. This recording method is known in the art as "serpentine recording".
Modern streaming tape drives are typically driven from 25 to 110 inches per second with recording densities of 10,000 flux reversals per inch or more, resulting in maximum data transfer rates above 90,000 bytes per second.
Normally, as with the MFM disk recording convention known in the art, magnetic information is recorded on the tape as a series of magnetic domains. The domains are produced by providing an appropriate write current to a magnetic write head. A blank tape is therefore equivalent to a tape having prerecorded "no information" at each data location. MFM (Modified Frequency Modulation) recording is a method of recording where a "one" (1) is represented by a flux transition in the middle of the bit cell and a "zero" (0) is represented by the absence of flux transition. A clock flux transition is written at the end of a bit cell containing a "zero" followed by a "zero".
The related art is replete with mechanisms for reading and interpreting the stream of output data from a streaming cartridge tape drive. The MFM flux transition convention discussed above and most other recording methods suffer from a variety of imperfections arising from mechanical, electronic, magnetic and user interface characteristics. One of the important new problems encountered with streaming cartridge tape drives arises from the user requirement for such drives to operate directly with floppy disk controllers in desk-top computer systems. This requires the streaming tape drive to provide an output data stream that meets the electrical and logical specifications expected for modern floppy disk drives.
Modern floppy disk controller designs tend to rely on recent improvements in floppy disk drive technology. These improvements include disk drive mechanical changes that have significantly reduced the instantaneous speed variation (ISV) errors in floppy disk drive output data streams. ISV in a floppy disk environment arises primarily from (a) fixed speed offset and (b) low-frequency periodic speed variation from mechanical eccentricities. The modern floppy disk controller is typically tuned to enhance floppy disk data recovery by narrowing the bandwidth of the data synchronizer/separator element to take advantage of the improved ISV expected from the modern floppy disk mechanism.
This situation creates a new problem for streaming cartridge tape drives that are intended for use with a floppy disk controller as the computer system interface device. The tape cartridge itself is a primary source of the problem because it introduces a significant amount of high frequency ISV that must be isolated by the interface electronics to permit proper data recovery. The modern floppy disk controller cannot pass the high frequency ISV from tape cartridges because of its narrower data synchronizer/separator bandwidth.
In U.S. Pat. No. 4,644,420, William A. Buchan, discloses a circuit and method for tracking the output data stream from a cartridge tape drive independently of the data pattern in the tape. Buchan notes that the typical phase-locked loop tape reading circuit adjusts a read window to compensate for tape cartridge speed variations by matching the read data in a phase comparator with an adjustable output frequency from a voltage-controlled oscillator (VCO). This procedure creates a problem whenever a long series of zeros passes over the read head because the VCO expects to be constantly adjusted to remove the time displacement error between the VCO frequency and the read data transfer rate. If an instantaneous speed variation occurs during a long string of binary zeros, the VCO time displacement error cannot be detected until a binary one terminates the string of zeros. Buchan also notes that the earlier attempts to solve the problem by enhancing the phase-locked loop circuit gain introduced more problems than they solved because of bandwidth limitations. Buchan teaches a solution to the problem that adds a sample-and-hold circuit to the phase-locked loop to store the time displacement error during the periods between flux transitions but does not consider the separate problem of high-frequency ISV.
In U.S. Pat. No. 4,837,643, James V. Tierney III, discloses a data smoother for a streaming cartridge tape drive that includes a refined method for adjusting the phase of a clock signal extracted from the output data stream. Tierney notes that all streaming cartridge tape drives in the art include some type of data smoother to reduce the high frequency variations in the output data stream induced by non-uniform spacing of magnetic transitions on the tape ("peak shift"). Tierney introduces a phase-locked loop and a frequency comparator circuit to permit continuous and infinitesimal adjustments to the time displacement of the clock signal extracted from the output data stream, thereby solving the problem of inadequate clock phase precision. Tierney also removes higher frequency "peak shift" components from the output data stream by adding a low pass filter to the reconstructed data clock signal circuit, but this solution does nothing to remove ISV modulation from the data stream.
In U.S. Pat. No. 5,003,408, Richard A. Farkas et al, disclose a method and apparatus for removing data stream variations from streaming cartridge tape drive signals that uses frequency-modulation techniques to remodulate the raw data stream, thereby removing the effects of ISV from the data. To accomplish this, Farkas et al use a pulse-width modulator that operates on the data stream pulses under the control of an error signal produced through a phase-locked loop that contains a low-pass loop filter for removing the higher frequency FM arising from the ISV generated in the tape cartridge mechanism.
Thus, Farkas et al, use the Buchan phase-locked loop concept for data separation and generation of a clock signal together with a new concept for filtering the higher frequencies from the clock signal. However, Farkas et al. do not suggest how their technique can cope with ISVs that are so large that they cause a "cell slippage". That is, the Farkas et al. technique works well within a single data bit cell but cannot detect nor correct cell slippage arising from very large ISVs because the intermediate to lower frequencies are also removed.
As is known in the art, the high-frequency tape cartridge ISV components that are most troublesome are those above 7 KHz caused by longitudinal tape tension changes arising from microscopic slippages in the tape cartridge tensioning mechanism. The exact ISV frequency is cartridge dependent, but a 7 KHz frequency is the typical lower limit of the tension slippage induced ISV. ISV amplitude is specified by the cartridge manufacturer to be less than five percent (peak) of the tape speed.
Because of the recent changes in floppy disk controller ISV bandwidth specifications, there is a strongly-felt need for a solution to the high-frequency ISV problem that will permit existing cartridge tape drives to interface directly with the newer floppy disk controllers. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.